In four-cycle engines, blow-by gas, which is a gas that leaked from the combustion chamber through a gap between the cylinder wall and the piston, is generated in the crank chamber, and to send the blow-by gas back to the combustion chamber, a breather device for guiding the blow-by gas to the intake system is provided. The blow-by gas contains oil mist, and if the oil is mixed into the intake air, the emission will be deteriorated. Therefore, a breather chamber that communicates with the crank chamber is provided integrally with the crankcase to separate the oil mist from the blow-by gas at the breather chamber, whereby the mixture of oil into the intake air is prevented (see JP2006-200472A, for example).
However, in some cases, merely providing the breather chamber may not achieve sufficient oil separation. In such cases, it may be conceived to provide a filter device for oil separation in a part of a breather passage that connects the breather chamber with the intake system. However, in such an arrangement, in a cold state operation such as when the engine is started in a low temperature environment or when the engine is operated in a frigid environment, the viscosity of the oil is increased due to the low temperature and hence the oil tends to adhere to and clog the filter. To solve such a problem, it may be conceived to wrap the filter device and/or the pipe forming a part of the breather passage extending from the breather chamber to the filter device with thermal insulation material, to thereby suppress dissipation of heat that these members have absorbed from the blow-by gas. However, such an arrangement would increase the amount of material used and the number of assembly steps, and in addition, would increase the size of the breather device.